Apologies if this has been asked before (I did check and I believe it wasn't). I have a question about the particle/wave duality of photons (or other particles). Depending on what and how we measure the photon turns out to be either a wave or a particle. Recently I saw some web page (and I can't remember where) that maybe neither is true. Both the wave perception and the particle perception are just that: perceptions, strengthened by our perhaps limited ability to observe. What if the reality of the photon is something else, something "on top" of our two perceptions??
Could someone direct me to a site that could tell me more about it (or maybe debunk the whole idea?)?

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This is the idea: when you see the high school equations that describe a parabolic motion, you consequently visualize a flying stone in your mind. Physics does not care about what you imagine, but rather it deals with the equations and their ability to tell you the distance the stone will reach. Because the equations were invented and tuned for that purpose, the numeric results of that mathematical tricks are equal to what happens in nature.

Around the 1930s, some men eventually managed to build a consistent sets of mathematical tricks that correctly give the same results as nature, when dealing with a wide range of problems for which the known theories had failed. It is known as Quantum Mechanics. But, unlike the high school parabolic shot, the equations of QM are there but nobody can imagine a graphical representation.

Whatever it may be happening down there, there is no possible way of depicting it in your human brain. It is just so. You may spend your life thinking about a wave as a transparent diffuse entity that suddenly feels observed and transforms into a tiny hard ball... But the result will be nothing but a headache. With Quantum Mechanics you see the equations and check the result, but there is never a parallel mental image of a flying thing or whatever.

There is however good news, because the most basic of the theory is not difficult to grasp. But bear in mind that it consists on math (by the way, Physics may be well defined as the science that invents mathematical models that resemble the behavior of nature). Google for a small collections of video lectures called "Quantum Entanglements 1" by Susskind. After that, the third volume of the Feynman Lectures is a good choice (but that requires more work).

Of course, there is room for much more than pure math alone, but that room is only accessible in a mentally healthy way after you know the math. With no knowledge of the math, words about Quantum Mechanics are meaningless, and intuitive ideas are unavoidable wrong. Whatever meaning you give to the words "wave" and "particle", there is no way to combine them in a satisfactory explanation without at least some elementary knowledge of Quantum Mechanics.

The statement "Depending on what and how we measure the photon turns out to be either a wave or a particle." is quite old-fashioned, and unhelpful, in my opinion.

Let's consider an electromagnetic wave of frequence $\nu$. Classically, this wave can carry an arbitrarily small amount of energy, i.e., the electromagnetic field can have arbitrarily small wiggles in it, with frequency $\nu$. But this classical picture is wrong: quantum mechanics tells us that a wave of frequency $\nu$ can only have energies $n h \nu$, where $n$ is a non-negative integer and $h$ is Planck's constant. We say that "$n$ photons are present".

Suppose we pass the electromagnetic wave through a narrow slit. Classically, we expect diffraction, and quantum mechanically... exactly the same! All that changed with quantum mechanics is the possible values of the energy contained in the wave. So our $n$ photons 'behave like a wave'.

Now imagine we have an apparatus (perhaps a photographic film) set up to detect the electromagnetic wave. Classically, any amount of energy could be deposited anywhere on the film, but quantum mechanically, we know that absorption occurs via interactions with single atoms/molecules/whatever. Since the electromagnetic field can only carry discrete amounts of energy, and energy is conserved, it must be absorbed in 'units' of $h\nu$. In this sense, it looks like individual particles impinging on the apparatus, each of energy $h\nu$.

I'm going to challenge the suggestionn that absorption of light works by the interaction of single photons with single molecules. In my blog article on Quantum Siphoning, I show how you can understand the process as the interaction of a classical e-m wave with an ensemble of atoms in the photosensitive crystal. You can read the article here: marty-green.blogspot.ca/2010/03/quantum-siphoning.html
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Marty GreenNov 7 '12 at 16:46

Your reasoning contradicts experimental results, not least of which is the photoelectric effect.
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RhysNov 7 '12 at 18:13

I really posted my link because it specifically addressed your point about the photographic film. I wonder why it is necessary for you to bring up the photoelectric effect to counter my reasoning? My article deals with the photographic plate, not the photoelectric effect.
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Marty GreenNov 7 '12 at 18:46

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The principles are exactly the same, and your link only serves to confuse those who are trying to learn quantum mechanics.
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RhysNov 7 '12 at 19:47

Assuming you believe quantum field theory, specifically quantum electrodynamics, a photon is neither a particle nor a wave, so your web page is correct in this respect though I hesitate to give it too much credit without seeing its reasoning.

A photon is an excitation in the photon quantum field. It can interact in ways that resemble a particle and it can interact in ways that resemble a wave, but is incorrect to say it is a particle or a wave or some mixture of the two. It is an excitation in a quantum field.

+1 for "a photon is neither a particle nor a wave." It's best not to look at those tiny, tiny things described by QM (photons, electrons, quarks, etc.) as sometimes a particle, sometimes a wave. They are something different. Looking at them as waves or particles runs into the American football / kidnapping an adult / Dirac delta function / cartesian tensor / dwarf planet / Red Sea / red herring problem. Red herrings are very rarely red and are very rarely fishes.
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David HammenJul 23 '14 at 11:26

Dear John, (+1) May I ask you to please elaborate a bit on your remark "believe" QFT. Especially in view of the above mentioned notion of mathematical modeling. Is it a question of the accuracy of modeling, or the presumed physics that is being modeled? Also (aside from gravity), is there a large degree of skepticism regarding QFT. (Or maybe, since it as yet has not resolved gravity, does that render QFT as only as good as can be done for now, but still possibly in question?) Thanks very much - sorry if this is an imposition. Regards,
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user41976Aug 7 '14 at 15:41

@Andrew: you may be reading more into my wording than I intended. QFT has proven over and over again to be an exquisitely accurate way of calculating the behaviour of quantum systems and no-one seriously doubts it. Even newer theories like string theory reproduce QFT as a low energy limit. However a quantum field is a very odd object indeed, and its physical meaning leaves many of us scratching our heads nervously.
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John RennieAug 7 '14 at 15:58

Photons, electrons, quarks,... are always particles [*] and never are waves. As the CERN site correctly notes everything in the universe is made of particles. To be concrete the photon is a boson particle.

The source of the wave-particle duality misconception is pointed out in Ballentine recent textbook on quantum mechanics:

Are "particles” really ”waves”? In the early experiments, the diffraction patterns were detected holistically by means of a photographic plate, which could not detect individual
particles. As a result, the notion grew that particle and wave properties were mutually incompatible, or complementary, in the sense that different measurement apparatuses would
be required to observe them. That idea, however, was only an unfortunate generalization from a technological limitation. Today it is possible to detect the arrival of individual
electrons, and to see the diffraction pattern emerge as a statistical pattern made up of many small spots (Tonomura et al., 1989).

This is in regard to shooting single photons or electrons through a double-slit and the result that show the duality of particle and waveform properties coexisting.

Take Schrodinger's Equation and wherever a term is used, break it down into Mathematics Series functions of units representing space (length) and time (seconds).

Cross-reference Physics "Constants" to these Mathematics Series.

The Constants represent Relativistic Singularities and they are proposed to be formed from differences in non-relativistic quantum causality systems. Relativistic Systems floating upon moderated systems of quantum causality.

A Non-Relativistic Singularity is a constant, but it is not referenced directly to anything observable, only indirectly through the connections with the Relativity Singularities (relativistic physics constants).